I Have A Chawauwa (can't Spell It)/ Terrier Mix. He's

I have a chawauwa (can't spell it)/ terrier mix. He's been sick for five days. Took him to vet and waiting on lab results. He is lethargic, no appetite, rapid breathing, and left back leg seems to give out on him at times. Vet put him on antibiotic and Tramadol-p for stuffy nose three days ago. Still has above symptoms. He also walks with his tail tucked between back legs. What's wrong with my baby?